The invention relates to the field of high-voltage (HV) cables, such as are used for example for connecting under compression to X-ray emitter units and the like. In particular, but not exclusively, the invention relates to HV cables which comprise means for indicating the current amount of spring compression in the connection.
Cables for connecting voltage sources operating at potential differences of hundreds or even thousands of kilovolts requite high performance insulation and reliable, durable connections. Such a cable may typically comprise a substantially straight central conductor, also known as the lead wire, which is usually flexible, to allow a certain flexing of the cable, encapsulated in a thick, high performance insulator. The end of the cable may typically be provided with an insulated male conical plug which is designed to be inserted into a corresponding female conical socket of a high voltage device. One or both of the conical ping and socket typically comprise a compressible outer layer of insulating material such that, when the plug and socket are pressed together, the male and female insulating parts mate to provide a high level of insulation, even at their physical interface. The plug and socket must be kept under sufficient compression for the connection to retain its insulation integrity. This compression can be achieved by means of a compressible elastic element such as a coil spring, which is compressed during the connection process and remains compressed until the connection is released. The amount of force to be exerted on the end of the cable can be set by compressing the spring a certain distance at the time of connection, in a process commonly referred to as “gapping”. This term refers to the setting of the spring compression by adjusting a gap between a spring-compression plate and the socket.
The elastic insulation materials may expand and contract with fluctuations in temperature, and the materials may gradually soften and yield with time, especially if the device is regularly operating at high temperatures. The compression spring may extend or contract slightly to allow for such short-term expansions and contractions, and to maintain the compression force on the connector. As the insulation material gradually gives, the spring gradually extends, with the result that the force on the connector gradually wanes, and the likelihood of an unwanted electrical discharge through the insulator interface increases as a consequence. For this reason, this type of connector requires regular “re-gapping”. An adjustment means is provided for re-adjusting the amount of compression in the spring and thereby resetting the compression force at the interface between the connector insulator cones.
European patent application EP1646268 (Yxlon International X-Ray GmbH) describes an example of such a connection arrangement. In the arrangement of EP1646268, a separate clamping collar is used to clamp the end of the cable to the connector of the high-voltage device. The collar contains a pre-compressed spring arranged such that, when the clamping force between the collar and the high-voltage device exceeds the compression on the spring, any further increase in the clamping force results in an increased compression of the spring. In this way, the risk of over-tightening the connection can be reduced, and the spring can maintain the force between, the mating conical insulators when the insulation begins to give. The spring is held in compression by a plate or housing of the collar, and provides a force which urges a cable-engaging element towards the high-voltage device when the sprung collar is fitted to the cable. The cable is provided with a collar-engaging element, for example having an external thread. The removable sprang collar is provided with a cable-engaging element which is longitudinally mobile within the collar housing. The cable-engaging element of the sprung collar has an internal thread for engaging with the thread of the collar-engaging element of the cable.
When engaged with the cable and screwed to the high-voltage device, the spring in the collar pushes on the collar-engaging element of the cable, which is rigidly connected to the male conical part (the end portion) of the connector, thereby urging it into the female conical connector of the high-voltage device.
Gapping and re-gapping (ie adjusting the compression in the connector interface) can be carried out by adjusting the threaded engagement between the cable-engaging element of the sprung collar and the collar-engaging element of the cable, in order to adjust the relative positions of the two engaging elements.
If the cable must be removed or replaced, the separate clamping collar is removed, and can be re-used on the replacement cable. The sprang collar also includes an indicator peg, secured to the cable-engaging element. The indicator peg moves gradually along a slot in the outer housing of the sprung collar as the spring gradually extends over time. When the indicator peg of the collar has moved a certain distance relative to the housing, an operator can visually detect that the connection must be re-gapped. He or she then undoes the collar, adjusts the position of the collar relative to the cable (by turning the collar, for example, so as to rotate the threaded part of the collar relative to the threaded part of the cable), re-fits the collar on to the cable (or on to a replacement cable if the cable must be replaced), and re-tightens the clamping screws in order to clamp the connection once more under compression.
Such re-gapping is typically carried out at regular service intervals (every few months, for example) by specialist operatives. The compression in the connector can be approximately gauged by observing the position of the indicator peg relative to markings on the housing. However, such indicators are inaccurate. Furthermore, visual inspection requires that an operator be in close proximity with the device. Since this type of connector may be used with equipment such as X-ray machines, it is often not possible to observe the machine while it is operating. To this end, EP1646268, also proposes the use of electrical or magnetic switches for signalling (on a remote display, for example) that re-gapping of the connector is required (ie when the spring has extended by a predetermined distance). The particular sensors required are integrated into the removable collar, along with the spring. As an alternative, the re-gapping alert can be triggered by means of a pressure sensor which detects when the force provided by the spring falls below a certain pre-settable threshold. Again, the pressure sensor is integrated into the removable collar.
In the variants disclosed in EP1646268, the purpose of the re-gapping sensor is to indicate the compression of the spring between the housing and the cable-clamping element of the sprung collar.
If an electrical or magnetic or pressure sensor is used in the arrangement of EP1646268, then it must be set to trigger well before re-gapping is required, in order to allow a margin of time in which to prepare and carry out the operation. In many instances, therefore, re-gapping will be carried out before the operation is truly necessary.
The clamping collar described in EP1646268 may be used with different types of cable. Each time a cable is replaced, the clamping collar, with its integrated spring and re-gapping indicator, is re-used to damp the new cable. However, the spring and re-gapping indicator may not be optimum for use with the replacement cable, which may result in incorrect gapping or a misleading re-gapping indication.